Far field signal simulation utilizing cylindrical arrays

ABSTRACT

A performance monitoring test set having a cylindrical near field multi-stave transducer array which simulates far field conditions. The multi-stave array is housed in a cylindrical test tank with appropriate shading and phasing of the staves to produce a constant pressure volume in the near field. The test tank system permits testing of a sonar system&#39;&#39;s range and bearing accuracy, beam uniformity, minimum detectable signal level and transmitter response.

IJnited States Patent 1 OBrien et al.

[ June 19, 1973 FAR FIELD SIGNAL SIMULATION UTILIZING CYLINDRICAL ARRAYS[75] Inventors: Paul J. OBrien; Harold F. Messias,

both of Scituate, Mass.

[73] Assignee: The United States of America as represented by theSecretary of the Navy, Washington, DC.

[22] Filed: June 15, 1971 [21] Appl. No.: 154,240

[52] U.S. Cl. 340/9 [51] Int. Cl. H04b 13/00 [58] Field of Search 340/5,6, 6 S, 8,

[56] References Cited i UNITED STATES PATENTS 3,364,461 1/1968 Trott340/6 S 2,925,581 2/1960 Hackley et al.... 340/5 R 3,368,190 2/1968Wilson et a1... 340/6 S 3,585,579 6/1971 Dorr 340/9 PrimaryExaminerBenjamin A. Borchelt Assistant Examiner-H. .1. Tudor Attorney-R.S. Sciascia and Thomas O.Wats0n, Jr.

ABSTRACT 6 Claims, 3 Drawing Figures PATENIED 9.975 3. 740. 707

X=PLANE ARRAY SHADING COEFFICIENT O=CYLINDRICAL ARRAY SHADINGCOEFFICIENT delay: distance velocity 2O NEAR FIELD ARRAY ELEMENTS SPACEDEVERY I5 DEGREES FIG. 2

AN/AQS IO HYDROPHONE ARRAY 5 ELEMENTS PER BEAM IFT.

NEAR FIELD TEST ARRAY IO STAVES 0.5 PER BEAM 1 FT. FT. 0.5 0.5 INVENTORSJim 0. ((1%).

ATTORNEY STATEMENT OF GOVERNMENT INTEREST The invention described hereinmay be manufactured and used by or for the Government of the UnitedStates of America for governmental purposes without the payment of anyroyalties thereon or therefor.

BACKGROUND OF THE INVENTION The invention relates to a system formonitoring the performance characteristics of sonar systems. It has theunique advantages of providing a detailed analysis of the sonar systemstatus while in a static operating condition, (e.g. actual test siteinstallation, aboard ship).

In the past sonar system performance has been tested by two differentmeans. In one type of system, signals were pre-recorded and played backthrough the sonar system to simulate the actions of a real target. Thesystem provides artificial, electrically generated simulation and doesnot adequately measure performance characteristics under actualoperational acoustic conditions.

Another prior art system for determining the performance of a sonarsystem compares the response of a standard reference channel with theresponse of the sonar system under test to a sound generated at adistance great enough to produce planar waves at the respectivereceiving hydrophones. The disadvantages of this system is that itrequires long test distances as well as elaborate calibration testsites. As a result, an expensive downtime of the system is necessary.

To measure a sonar transducer in the near-field a test array must beprovided which will produce a constant pressure volume at the desiredfrequencies over the active face of the sonar transducer under test. W..I. Trott in'U.S. Pat. No. 3,393,400 dicloses a method and apparatus forcreating a constant pressure plane wave under near field conditions bythe use of a planar transducer array. A technique is disclosed fordetermining the shading and phasing required for producing such a wavefront and a method for using such a planar array to calibrate atransducer. The present invention utilizes this concept to produce theperformance monitoring test system which simulates actual operationalacoustic conditions without the attendant disadvantages mentionedpreviously.

SUMMARY OF THE INVENTION The general purpose of the present invention isto provide a system in which all transducer parameters can be measuredin the near field by simulation of far field conditions. Far fieldconditions are simulated by a cylindrically configured array of aplurality of small omni-directional transducer elements, each of whichis smaller than a wavelength, spaced close together which when energizedproduce a constant pressure volume in the near-field. The array providesan accurate measure of beam uniformity, minimum detectable signal level,transmitter response and range and bearing accuracy. The cylindricalarray obviates the need for long test distances, elaborate calibrationtest sites and extensive downtime" of a sonar system.

OBJECTS OF THE INVENTION A primary object of the present invention is toprovide a test system which accurately measures the operating parametersof a sonar system.

Another object is to simulate far field conditions in the near fieldregion.

A further object is to provide a transducer array which produces a nearfield pressure distribution which is constant.

Still another object is to provide a sonar performance monitoring testset which closely approximates actual operational acoustic conditions.

A still further object is to provide a sonar performance monitoring testset considerably reduced in test system complexity.

Other objects and advantages will become readily apparant from thefollowing detailed description of the invention when considered inconjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a typical stave assemblyfor use in the cylindrical array.

FIG. 2 represents the shading function and the phasing of a 10 elementsection of a stave cylindrical array.

FIG. 3 illustrates the constant pressure volume produced by a 10 elementsection of a 24 stave cylindrical array.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT To measure anytransducer in the near-field, a test array must be designed which willprovide a constant pressure volume at the desired frequencies and overthe active face f the transducer under test. The design parameters ofthis array are, therefore, the operating frequency range and the testdimensions of the transducer under test. Also, the transducer elementswhich make up the measuring array should be spaced a sufficient distanceapart so that the test array will in effect be acoustically transparentover the entire operating frequency range. The minimum constant pressurevolume developed by the array is obtained at the lower frequency limit.This imposes the minimum size requirements on the array. The maximumspacing of the individual elements in the array for adequate support ofthe sound field is 0.8 A. Where is the wavelength of the operatingfrequency. This requirement determines the minimum number of elementswhich must be used in the array.

Referring now to FIG. 1 there is shown a typical stave assembly 10 foruse in the cylindrical test array. The assembly is comprised of aplurality of small ceramic transducer elements 12 spaced a predetermineddistance apart. The transducer elements may, for example, be PZT-4ceramic cylinders sealed with glass to metal end caps. The element toelement spacing as well as element size is selected to assure that thearray is acoustically transparent at the operating frequency. Thetransducer elements are connected in parallel with appropriate amplitudeshading elements 14 respectively coupled in series with the parallelenergized elements. The amplitude shading was accomplished withcapacitors and etching of the element conductive surface as is known inthe art.

The stave 16 itself is a hollow neoprene tube substantially larger indiameter than the transducer and shading elements. theparallel-connected transducer and shading elements are inserted in thetube which is evacuated, filled with oil 18 and capped at each end.

The test array consists of a plurality of staves equally spaced on acircle of predetermined diameter. The diameter of the circle ispreselected to provide a cylindrical array which is acousticallytransparent at the operating frequency.

A typical array of the present invention constructed for testing anAN/AQS-lO sonar system consisted of 24 staves equally spaced on a 25inch diameter circle, parallel to one another and concentricallyarranged about the AN AQS-lO hydrophone. This resulted in a 4 inchstand-off distance between the test array and the AN- lAQS-lOhydrophone. The staves are horizontally shaded and connected in parallelto form the test array. A set of adjacent staves will be in operation atany one time.

Each stave of the test array for the AN/AQS-lO contains 19 small ceramictransducer elements spaced 4 inches apart for a total of 456 elements inthe array. The transducer elements are PZT-4 ceramic cylinders measuringone-half inch by one-fourth inch. The element to element spacing, aswell as the element size, assure transparency of the array at anoperating frequency of IOkHz.

FIG. 2 presents the shading function used for the AN- /AQS1O test array.The relative position of each stave in the array is indicated at 20. Thehorizontal and vertical shading coefficients for the cylindrical testarray were derived by projecting the relative stave position in thearray on to a modified Gaussian shading function which is obtained byusing the binomial expansion coefficient.

Also, presented in FIG. 2 is the phase shift introduced into each stave.Since only 10 staves are in operation at any one time the amount ofphasing is determined by the relative distance 22 of each stave from aplane drawn tangent to the center of the ten stave section.

The resulting cylindrical array exhibits a response similar to that of acomparable near-field planar array. The acoustic field developed by a 10stave section of the array is plotted as a pressure contour variation inFIG. 3. By inspection of this figure, it is apparent that a sufficientplane wave volume is present to enclose the sensitive receiving apertureof the AN/AQS-IO hydrophone and therefore a far field effect is obtainedin the near-field region.

The test array is used in conjunction with a conventional tank" in whicha particular sonar system to be tested is placed, thereby providing anoverall test tank system.

To test the AN/AQS-IO sonar system a cylindrical test tank is fabricatedfrom 0.25 inch steel. The tank is constructed with a diameter sufficientto provide a clearance between the tank walls and the cylindrical arrayof approximately 8 inches. The interior surface of the test tank islined with sound absorbing rubber (SAPERT), approximately 0.28 inchesthick, which is cemented to the tank. This results in a moderatereduction in the intensity of the acoustic reflection.

OPERATION To a test a sonar system, a receiving hydrophone is placedinside the cylindrical test array as previously described. The testarray is driven with a transmit signal generated by a test set. Ahydrophone output signal is returned through the test set and ismetered. By calibrating the system before testing, comparative resultsof the sonar minimum detectable signal level (MDS) and receiving beam tobeam uniformity will be indicated on a test set meter.

As discussed previously, with respect to the test array for testing theAN/AQS-IO sonar system, a set of 10 staves of the 24 stave cylindricaltest array are in operation at any one time. The spacing of the teststaves about the AN/AQS-IO hydrophone is such that a receiver signallevel may be monitored every 15 by switching the 10 stave section onestave at a time about the array. Uniformity among beams thus can becompared as well as discreet representation of the far field beampattern.

Minimum detactable signal (MDS) level for each receiving beam can bedetermined by utilization of a transmit signal attenuator. MDS is thatlevel producing a three DB increase over the level produced by noisewith the transmit attenuator at its maximum position. By adjusting thesignal level which is transmitted by the test staves until it is equalto the background noise inherent to the beam under test, an accuratemeasure of this system parameter can be made.

Similarly by reciprocity a sonar transmitter response can be monitoredat the various power levels of the sonar projector in the same manner asthe receiving response. The transmitting beam pattern can thus bemonitored every 15. A measure of the transmitted power output of thesonar under test is made by monitoring the signal level received by thetest array and compensating for the gains and losses through the system.

System range and bearing accuracy can be checked by injecting a signal,which is keyed to the sonar transmitter, every 15 and at every rangescale. This is accomplished by transmitting a 3.5 or 35 millisecondpulse, the repetition rate of which is determined by the range scaleselected on the sonar system control set. Target range is simulated byintroducing a variably adjustable delay in the transmit pulse. The delaysimulates the two-way travel time of a pulse in the water. No power isdelivered to the sonar projector when a simulated target is being used.

Sonar system performance test data can also be obtained throughoperation of a single test stave. The function of the single stave testis identical to that of a multiple stave system to check out theoperational performance of a sonar system. Because of the nature of thissystem, it is capable of performing this function with a reduction intest system complexity (e.g. elimination of delay lines, severalpreamplifiers, switches, etc.).

Beam uniformity, minimum detectable signal, transmitter response andrange and bearing accuracy tests are performed in the same manner aswith the multiple stave test system except only one stave at a time isenergized. The tests are performed by simply switching one stave at atime around the array. The acoustic field produced, instead of being aplane Wave, is essentially a cylindrical wave. The cylindrical wave isdue to the vertical shading inherent in the stave, is constant inpressure and phase in the vertical dimension, but possesses a distinctcurvature in the horizontal dimension. Thus,

though the single stave system results in reduced test systemcomplexity, a sacrafice results from the consequent loss in the absoluteindication of system performance. 7

Thus the present invention provides an array which accurately simulatesfar field conditions in the near field. The cylindrical array provides aconstant pressure near field which can be used to test the operationalcharacteristics of a sonar system.

Obviously many modifications and variations of the present invention arepossible in light of the above teachings The invention is in no waylimited by the embodiment and functions described, which were givenmerely by way of example. It is therefore to be understood that withinthe scope of the appended claims the invention may be practicedotherwise than asspecifically described.

What is claimed is: 1. An electroacoustic transducer array for testing asonar system comprising: a plurality of staves; a plurality oftransducer elements within each stave; each of said staves containingthe same number of transducers and the transducers within each stavebeing spaced the same distance apart; said plurality of staves arrangedon a circle of predetermined diameter such that the transducers arealigned and equally spaced both vertically and cir- I preselected size.

4. The electroacoustic transducer array of claim 1 wherein each stavecomprises a sealed, hollow neoprene tube containing oil.

5. The electroacoustic transducer array of claim 1 wherein said shadingmeans includes a capacitor electrically coupled in series with eachtransducer.

6. The electroacoustic transducer array of claim 2 .wherein thecylindrical array comprises:

24 staves arranged on a circle of approximately 25 inches in diameter;

said staves contain nineteen transducer elements spaced 4 inches apart;and

said transducer elements consist of inch by A inch ceramic cylinders.

1. An electroacoustic transducer array for testing a sonar systemcomprising: a plurality of staves; a plurality of transducer elementswithin each stave; each of said staves containing the same number oftransducers and the transducers within each stave being spaced the samedistance apart; said plurality of staves arranged on a circle ofpredetermined diameter such that the transducers are aligned and equallyspaced both vertically and circumferentially whereby the cylindricalarray is acoustically transparent at the operating frequency; and meansfor shading each of said transducers whereby the cylindrical array has aconstant pressure volume near-field.
 2. The electroacoustic transudcerarray of claim 1 wherein said transducers are electrically coupled inparallel.
 3. The electroacoustic transducer array of claim 2 whereinsaid transducers are ceramic cylinders of a preselected size.
 4. Theelectroacoustic transducer array of claim 1 wherein each stave comprisesa sealed, hollow neoprene tube containing oil.
 5. The electroacoustictRansducer array of claim 1 wherein said shading means includes acapacitor electrically coupled in series with each transducer.
 6. Theelectroacoustic transducer array of claim 2 wherein the cylindricalarray comprises: 24 staves arranged on a circle of approximately 25inches in diameter; said staves contain nineteen transducer elementsspaced 4 inches apart; and said transducer elements consist of 1/2 inchby 1/4 inch ceramic cylinders.